The invention relates to the field of communications, in particular to wavelength filters including those used in wavelength division multiplexed optical communication systems, and more particularly to a means to determine the status and integrity of such wavelength filters.
Optical communications systems are a substantial and fast-growing constituent of communication networks. The expression xe2x80x9coptical communication systemxe2x80x9d, as used herein, relates to any system which uses optical signals to convey information. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.). Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from a plurality of sources, time-division multiplexing (TDM) is frequently employed. In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal from one of the signal sources being reconstructed from the portions of the signals collected from the relevant time slots. While this is a useful technique for carrying information from a plurality of sources on a single channel, its capacity is limited by fibre dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g. by laying more fibre optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) is now used for increasing the capacity of existing fibre optic networks. In a WDM system a plurality of optical signal channels are carried over a single waveguide, each channel being assigned a particular part of the spectrum. Ideally each channel will be allocated to a wavelength band centered upon a single wavelength. In practice, due to the shortcomings of available sources and spectral broadening due to the modulation on the carrier and due to the dispersion and propagation of transmission media, each signal channel will spread across the spectrum to a greater or lesser extent. References herein to xe2x80x9ca wavelengthxe2x80x9d are to be interpreted accordingly.
Optical fibre networks have been explored to permit the transfer of optical signals carrying WDM channels (WDM signals) bearing analogue or digital data, from one optical fibre in one loop, ring, cell of a mesh or line of a network to a different loop, ring, cell of a mesh or line of the network directly, in optical form, without the need to convert the signals into electrical form at interconnection points of the network. These interconnection points (or nodes) comprise optical add-drop multiplexers OADMs or optical cross connects OXCs.
Several methods to achieve optical add drop multiplexing (or switching) and optical cross connect switching are described in the proceedings of the European Conference Optical Communications, September, 1998, Madrid, Spain and the Optical Fibre Conference, February 1998, USA.
A known method to achieve switching of a WDM signal from a set of WDM signals is to pass the set into a first port of an optical circulator. These WDM signals will exit the circulator at a second port. A series of tunable optical filters are positioned at the second port such that selected ones of the set of WDM signals are reflected by the series of filters back into the second port and other ones of the set are passed by the same series of filters. The reflected signals reenter the optical circulator at the second port and emerge at a third port.
A filter at the second port is required for each WDM signal. Switching is achieved by arranging that the filters may be de-tuned or adjusted by an amount comparable to the spectral width of the WDM signal. Hence, if a filter is arranged to normally reflect a particular WDM signal, tuning will cause it to pass that signal. Alternatively, the filter may be arranged to normally pass and on tuning to reflect a particular signal. This process allows any sub-set of WDM signals to be selected from the total set.
The large capacity of an optical fibre carrying WDM signals (e.g. 40 Gbit/s) and the potential value of the information content of the signals means that it is important to know which state a switch is in at any one time and that the switch device (i.e. the filter) will operate correctly when activated.
Thus there is a need for a method and a system to monitor the operation of the switchable filters in the switching and routing equipment of optical communications networks.
The present invention provides a system for checking the operation of a wavelength filter comprising a wavelength filter comprising a first selectively reflective element (SRE) for reflecting a first signal of a first wavelength and a second SRE for reflecting a second signal of a different wavelength; the system also comprising checking means for checking the operation of the wavelength filter by detecting the signal reflected by a particular one of the selectively reflective elements.
In a preferred embodiment the invention provides a system in which the particular one of the SREs is associated with a reflection wavelength band that lies outside of a data band and the other of the SREs is associated with a reflection wavelength band that lies inside of the data band.
In a further preferred embodiment the invention provides a system in which the filter comprises an adjusting means for adjusting the wavelength reflected by the first SRE and the wavelength reflected by the second SRE in which, in operation, adjustment of the wavelength reflected by the first SRE results in a corresponding adjustment of the wavelength reflected by the second SRE.
In a further, preferred embodiment, the invention provides a system in which the filter comprises an adjusting means for introducing a change in the wavelength reflected by the first SRE and automatically introducing a corresponding change in the wavelength reflected by the second SRE.
The present invention also provides a method for checking the operation of a wavelength filter comprising a first selectively reflective element (SRE) for reflecting a first signal of a first wavelength and a second SRE for reflecting a second signal of a different wavelength; the method comprising the step of detecting the signal reflected by a particular one of the SREs.